This invention relates to launchers used to launch a pipeline pig into a pipeline system. More specifically, the invention relates to systems and methods of preventing flow blocking when launching a pig using an automated pig launcher.
Pipeline pigs perform various pipeline maintenance, cleaning and inspection operations while the pipeline continues to operate under pressure and transport product. The pig is introduced into the pipeline by way of a trap (the “pig launcher”) connected to the pipeline. Once the pig is placed inside the launcher, the closure door of the launcher is closed and pipeline flow is used to push the pig into and through the pipeline. The pig continues to travel through the pipeline until the pig reaches a receiving trap (the “pig catcher”).
Referring to FIG. 1, a prior art pig launching system uses gravity to launch a pig into a main pipeline. Pipeline product is diverted into and flows through the launcher when the kicker and isolation valves are opened and the mainline bypass valve is closed,
Pig launching and receiving systems vary from one to another depending on specific purpose and pipeline product. Variations include such things as placement of valves, length of the launch tube or barrel, type of closure door, and the means to move a pig into the reduced cross-section area of the barrel (e.g. gravity, screws).
Regardless of those variations, all launchers and receiver include the barrel which holds one or more pigs, a large branch connection which connects the barrel to the pipeline system, and a set of valves that divert and control flow through and to the launcher (see e.g. FIG. 1). The set of valves typically includes the mainline bypass valve, mainline trap or isolation valve, and trap kicker or kicker valve. When ready to launch a pig into the pipeline system, a Tee section diverts pipeline fluid flow into the end of the barrel, behind one or more of the pigs positioned in the barrel.
In order to launch itself properly into the pipeline, the pig must experience an adequate pressure differential across the pig body. Gravity assistance can be used to move the pig into a position to better accomplish this. Referring to FIG. 1, a prior art pig launching system uses gravity to launch a pig into a main pipeline. Pipeline product is diverted into and flows through the launcher when the kicker and isolation valves are opened and the mainline bypass valve is closed,
The ability to control bypass flow through the pig body, therefore, is critical to providing sufficient acceleration of the pig during its launch into the pipeline and controlling the speed of the pig as it travels through the pipeline. In many cases—such as those where minimal flow is available to push the pig out of the launcher and into the pipeline—it is desirable to have minimum to no bypass flow through the pig during the initial launch phase, that is, up until the time at which the pig passes the isolation valve.
U.S. Pat. No. 4,135,959 to Reese discloses monitoring the operating condition of a pig launcher to prevent opening of the launcher closure when there is a pressure in the launcher and to prevent flow into the launcher when the launcher closure is not in place. For example, the override maintaining the isolation valve in a closed position cannot be removed until the pressure in the barrel reaches a set point of about ½ to 1 psig.
Fully automated pig launchers have the potential to launch pigs in ways that manual and semi-automated pig launchers could not. For example, a fully automated system could modulate the mainline bypass valve to create differential pressure sufficient to launch a pig without the use of gravity assistance. However, this modulation could potentially block product flow in the pipeline by closing or partially closing the mainline bypass valve when the kicker or isolation valves are closed or partially closed. Therefore, a need exists for a system and method to prevent this flow blocking and assure adequate flow of pipeline product.